1. Field of the Invention
The present invention relates to a thermal head that is used in a printing unit of a printer.
2. Description of the Related Art
A thermal head 1, which is mounted on a printing unit of a printer, includes a head substrate 2 as shown in FIG. 7. A heat storage layer 3a, which is made of an insulating material such as glass, is formed on the head substrate 2. A partial glaze 3b, which is formed in a cylindrical shape, is formed at a part of the heat storage layer 3a. Heating resistor layers 4 are stacked on the heat storage layer 3 so as to have a predetermined width in a main scanning direction. A heat generating element 6 is formed of the heating resistor layers 4 and electrodes E that are formed on the heating resistor layers 4 and made of Al through which current flows. Further, a protective layer 11 is formed. The protective layer 11 is made of an abrasion-resistant material such as SiAlON or Ta2O5, covers the heating resistor layers 4 and the electrode layer E of the heat generating element 6, and protects the surfaces of the heating resistor layers and the electrode layer. A plurality of driver ICs 12 (see FIG. 8), which is aligned in a main scanning direction orthogonal to a recording sheet conveying direction (in a width direction of a recording sheet), is provided on the head substrate 2 or a printed-circuit board (not shown) that is closely provided. When being provided on the printed-circuit board, the driver ICs 12 are connected to the electrode layer E, which is formed on the head substrate 2, by wire bonding or the like.
Here, the heat storage layer 3 is a glaze layer formed on the head substrate 2, and is formed so as to extend in the main scanning direction. Further, the heating resistor layers 4 are partially formed on the heat storage layer 3, and are made of a cermet material such as Ta2N or Ta—SiO2. The electrode layer E includes individual electrodes 9 that are connected to one ends of the heating resistor layers 4 in a sub-scanning direction, and a common electrode 10 that is connected to the other ends of the heating resistor layers 4 in the sub-scanning direction.
Here, the individual electrodes 9 are electrodes that individually supply current to the respective heating resistor layers 4, and the common electrode 10 is an electrode that applies a common potential to the plurality of heating resistor layers 4. The individual electrodes 9 are formed of strip-shaped electrodes, which extend in a longitudinal direction of the heating resistor layer 4 and are formed of thin metal films as conductors, and are connected to terminals 12a of the plurality of driver ICs 12 that switches the electrical connection/disconnection of the corresponding individual electrodes 9.
Here, in the thermal head 1, the individual electrodes 9, which are connected to the terminals 12a of one driver ICs 12, are typically formed of a wiring pattern that spreads toward the corresponding heat generating elements 6 from the respective terminals 12a in the shape of a symmetrical fan due to various reasons, such as a resistance value and dimensional difference between the terminal and the heat generating element. That is, the wiring pattern of the individual electrodes 9 of the thermal head 1 is formed in a radial shape (in the shape of ribs of a fan) where the length of the individual electrode 9 disposed in the middle is shorter than those of the individual electrodes 9 connected to the end portions of each driver IC 12 as shown in FIG. 8.
In the thermal head 1, the variation of the resistance values of the individual electrodes 9, which are connected to the individual heat generating elements 6, affects the heat generation of the heat generating elements 6, generates unevenness in printing density, and causes a good printing result not to be obtained. Various correction methods have been proposed focusing on this (see Japanese Unexamined Patent Application Publication Nos. 2010-5794 and 62-282950).